Add tempest_tempest_conf_overrides to fs overrides

In order to move standalone scenario jobs, fs overrides job
definition contains tempest_extra_config,
https://review.openstack.org/#/c/648137/ in os_tempest provides
tempest_tempest_conf_overrides for the same. Since currently
we need to support both os_tempest and validate_tempest role
so adding the equivalent var in the job.

https://tree.taiga.io/project/tripleo-ci-board/task/902

Change-Id: I3ddac4da80ed17bf690fc7eb928ca6e762547d12

roles/run-test/tasks/main.yaml

@@ -19,6 +19,7 @@
         - 'dlrn_hash_tag'
         - 'dlrn_hash_tag_newest'
         - 'use_os_tempest'
+        - 'tempest_tempest_conf_overrides'
     fail:
       msg: "ERROR: {{ item }} is not overridable."
     when: item not in "{{ allowed_overrides }}"

zuul.d/standalone-jobs.yaml

@@ -543,3 +543,5 @@
       featureset_override:
         run_tempest: false
         use_os_tempest: true
+        tempest_tempest_conf_overrides:
+          auth.use_dynamic_credentials: true